1. Field of the Invention
The present invention relates to novel cyclic tertiary amine compounds and to organic electroluminescent devices (hereinafter abbreviated as “organic EL devices”) using the compounds.
2. Description of the Related Art
Recently, organic EL devices have been given much attention as a full color flat panel display of next generation and vigorous research and development thereon have been under way. The organic EL devices are injection type luminous devices that have two electrodes sandwiching a luminescent layer therebetween. They emit light when electrons and holes are injected into an organic luminescent layer to be recombined therein. The material that can be used includes low molecular weight materials and high molecular weight materials. Both of them are proved to give organic EL devices with high luminance.
Such organic EL devices include two types. One type uses an electron transport material containing a fluorescent dye disclosed by C. W. Tang, et al. (J. Appl. Phys., 65, 3610 (1989)) as a luminescent layer. Another type uses a fluorescent dye itself as a luminescent layer (for example, the device described in Appl. Phys. 27, L269 (1988)).
The type that uses a fluorescent dye as a luminescent layer is roughly divided into 3 sub-types. The first sub-type includes a three-layer structure having an electron transport layer and a hole transport layer sandwiching a luminescent layer therebetween. The second sub-type includes a two-layer structure having a hole transport layer and a luminescent layer laminated one on another, and the third sub-type includes a two-layer structure having an electron transport layer and a luminescent layer laminated one on another. These laminate structures are known to improve the luminous efficiency of organic EL devices.
The electron transport layer in the organic EL device of the above-mentioned structure contains an electron transport compound and has a function of transporting electrons injected from the cathode to the luminescent layer. The hole transport layer and hole injection layer are layers that contain hole transport compounds, respectively, and have functions of transporting holes injected from the anode to the luminescent layer. Interposition of the hole injection layer between the anode and the luminescent layer enables transport of many holes from the anode to the luminescent layer at a low electric field and further enables confinement of electrons injected from the electron transport layer or electron injection layer within the luminescent layer. This can give rise to organic EL devices having excellent luminescent ability such as increased luminous efficiency.
However, these organic EL devices do not have enough performance sufficient for practical use. The major reason therefor is insufficient durability of the material used, and in particular, poor durability of the hole transport material. Heterogeneous portion such as grain boundary, if any, in the organic layer in an organic EL device causes the concentration of the electric field in that portion, which may lead to the deterioration or breakage of the device. Therefore, the organic layer is used mostly in an amorphous state. Also, the hole transport property of the hole transport material was insufficient so that the luminous efficiency of the device was insufficient for practical use.
Although various materials centered on triphenylamine derivatives have been known as the hole transport material used in such organic EL devices as described above, there are only few materials that are suitable for practical use. For example, N,N′-diphenyl-N,N′-di(3-methylphenyl)-4,4′-diaminobiphenyl (hereinafter abbreviated as “TPD”) has been reported (Appl. Phys. Lett. 57, 6, 531 (1990)). However, this compound is poor in thermal stability and causes a problem in the service life of the device containing it. Many triphenylamine derivatives are disclosed in U.S. Pat. Nos. 5,047,687, 4,047,948 and 4,536,457, Japanese Patent Publication No. Hei 6-32307, Japanese Patent Application Laid-open Nos. Hei 5-234681, Hei 5-239455, Hei 8-87122 and Hei 8-259940. However, none of the compounds have sufficient properties.
Among the amine derivatives disclosed in Japanese Patent Application Laid-open Nos. Hei 4-308688 and Hei 6-1972-and Adv. Mater., 1994, 6, No.9, p.677, which are called starburst molecules by the authors based on the structures of the compounds, and also among the compounds disclosed in Japanese Patent Application Laid-open Nos. Hei 7-126226, Hei 7-126615, Hei 7-331238, Hei 7-97355, Hei 8-48656, Hei 8-100172 and Hei 9-194441 and J. Chem. Soc. Chem. Comm., 2175 (1996), there has been none that has practically indispensable property of having a long service life with high luminous efficiency. Org. Lett., 1, 13, 2057 (1999) discloses tetraazacyclophane derivatives. However, the literature contains no description that they are useful as a material for organic EL devices.
As described above, the hole transport materials used in conventional organic EL devices do not have practically sufficient performances and hence it has been desired to increase the efficiency and the service life of the organic EL devices by using excellent materials. Further, in the most organic EL devices, the luminescence is provided by the luminescent layer or electron transport layer separately arranged from a charge transport layer, but in few cases by the hole transport layer. The reason for this is considered that a luminescence color of the hole transport material itself and intensity of luminescence are regarded as important factors although there may also exist the problem of compatibility of the hole transport layer with the electron transport layer used simultaneously. It is anticipated that a layer that functions as a hole transport layer and at the same time as a luminescent layer, if any, will have a high practical value. However, few such materials have been known. In most cases, such materials have long emission wavelengths, and short wavelength emission cannot be obtained.